Metallic film resistors

ABSTRACT

Precision resistors are produced by depositing, at a temperature of from 40 to 90* C., a nickel-phosphorus film upon former rods of steatite from an electroless plating solution having a pH value of 5 to 7 and containing specified amounts of a water soluble hypophosphite, nickel sulfate, soluble pyrophosphate, a water soluble orthophosphate and a water soluble salt of ethylenediaminetetraacetic acid, the resulting nickel-phosphorus film resistor rods being subsequently subjected to heat stabilization; the temperature coefficient of resistance of the resulting precision resistors is in the vicinity of zero.

United States Patent [111 3,607,389

[72] Inventors Giovanni Canegallo; 3,172,074 3/1965 Drews et a]. 1 17/227 X Ubaldo Costa, both of Milli], Italy O H REFERENCES [21 1 Appl. No. 768,659 n Filed Oct. 18,1968 Silygarlts Proc. Amer. Electro. pl. Soc. Vol. 47 (1960) 451 Patented Sept. 21, 1971 {731 Assignee Soclete Elettroteenica Chlmica Italians Prim y Emminerwil|iam Jarvis Milan, Italy Attorney-Linton and Linton [32] Priority Oct. 21, 1967, Feb. 15,1968 [33] Italy [31] 21866A/67 and l208A/68 [54] METALLIC FILM RESISTORS 5 Chums, 2 Drawing as. ABSTRACT: Precision resistors are produced by depositing, at a temperature of from 40 to 90 C., a nickel-phosphorus [52] US. Cl. 117/227, film upon former rods of came from an (ecu-0165s plating H7/2l3' l W130i 338/308 solution having a pH value of 5 to 7 and containing specified 7/00 amounts of a water soluble hypophosphite, nickel sulfate, [50] Field of Search l l7/2l3, Soluble pyrophosphate a water soluble orthophosphate and a 2271 130 47 water soluble salt of ethylenediaminetetraacetic acid, the resulting nickel-phosphorus film resistor rods being sub- [56] References Cited sequently subjected to heat stabilization; the temperature UNlTED STATES PATENTS coefficient of resistance of the resulting precision resistors is NJXPPFQ' .31'L'fjllf27iiT. in the vicinity ofzero- T.C.R. (D.p.m/C)

10 100 1600 SURFACE RE SISTIVITY (ohms per square) PATENIEB $921 m T 6 (W 38E] FIG/I.

I] I III/I FIG.2.

1OO SURFACE RESISTIVITY m D are) b O O 5 O 6 GLEQE mOF AT TORNEYS METALLIC FILM RESISTORS The present invention relates to the production of electrical precision resistors and to a process for their manufacture by electroless deposition of a nickel-phosphorus film on a former, e.g. of porcelain, alumina, steatite or glass or other ceramic or plastics with an insulation resistance greater than ohm cm. at C. As is well known, in the manufacture of film resistors, the former must be sufficiently nonporous to prevent penetration of the chemicals used in the process described hereinafter into the body of the material and also have sufficient chemical stability to withstand the action of these chemicals. The term precision resistor" as used herein means one having a temperature coefficient of resistance (herein also referred to as TCR) within the range :50 parts per million (i.e. p.p.m.) per C., and a stability of resistance of better than 1 percent after 1,000 hours load endurance under conditions such that the maximum temperature of the resistor does not exceed 150 C. in the case of films with a surface resistivity up to 10 kilohms per square or 100 C. in the case of films with a surface resistivity greater than 10 kilohms per square.

Several chemical processes are known by means of which it is possible to deposit metallic films composed mainly of nickel, cobalt or iron on the surface of various objects, including plates or rods of glass, porcelain, steatite, synthetic resins, etc., without using electric current. Such procedures are described for example, in U.I(. Pat. specification No. 749,824 which makes use of the commonly known expedient of adjusting the pH value and temperature of the plating baths; the manufacture of electrical resistors in values of a few ohms to a few megohms is specified in said prior specification.

The films of the above-mentioned metals are obtained by chemical reduction from aqueous solutions, the essential constituents of which are hypophosphite ions and salts of the above mentioned metals. Some other substances are further added to these solutions in order to improve their stability and to obtain a more regular deposition of the metal. This type of deposition process is generally known as electroless plating.

It is known that substrates on which the metal film is to be deposited must be previously treated with aqueous solutions, e.g. ones containing so-called sensitizers and others containing so-called activators. The former generally consist of aqueous solutions of stannous chloride and the latter of very dilute solutions of chlorides of palladium or of other precious metals. The metal films which are so obtained adhere well to the surface of electric insulating bodies and can have surface resistivities varying, according to their thickness and composition, from 0.1 ohm per square to several hundred thousand ohms per square. They are therefore potentially useful for the preparation of resistors of the type normally used in electronic equipment.

Metal films obtained by chemical deposition with the above-mentioned solutions, in which the reducing agent is the hypophosphite ion, contain a percentage of phosphorus which may vary from a few per cent to 10 percent and more, so that these films may be considered as an alloy or solid solution consisting of one or more metals and phosphorus. Metal films obtained by these chemical reduction processes cannot however be used as such for producing precision electric resistors because their resistance and their TCR are very unstable and vary with time and according to the conditions under which they have to operate. In general, the TCR varies as the film thickness (and hence the surface resistivity) is varied by altering the deposition time in any given electroless plating bath. When a wide range of surface resistivities is being produced and a low TCR is required, in order to compensate for this change in TCR with resistivity, it is generally necessary to alter one or more parameters of the deposition process according to the value of surface resistivity which is required.

Several methods of achieving the required adjustment of the TCR are obvious from well established knowledge which is used in the manufacture of resistive films. Thus, nickel films which are thicker than about 150 A. are known to have a TCR greater than +1,000 p.p.m./C., and it is only the fact that the electroless nickel" films have appreciable phosphorus content which confers a much lower TCR to these films. It is a well established fact that the TCR of films of metallic elements progressively change from large positive values to small positive, zero or even negative values as the metal is converted to an alloy by the inclusion of another suitable element, e.g. phosphorus, in the film. It is also well known that the percentage of phosphorus can be increased by lowering the pH value of the metallizing solution and, to a more limited extent, the same result can be achieved under certain circumstances by lowering the temperature of deposition. However, adjustment of pH value has a major disadvantage as a method of controlling the percentage of phosphorus and hence the TCR. This disadvantage is due to the fact that the pH value drifts during plating in a manner which, under practical conditions, is not entirely predictable. Furthermore, it is very desirable to be able to select pH values for solutions from considerations other than the control of the T CR. These disadvantages will be demonstrated in the course of the description of the present invention which is intended to overcome or minimize these difficulties, and also other deficiencies previously encountered.

The present invention provides a process for the production of a nickel phosphorous film precision resistor which comprises:

i. effecting deposition on an activated, insulating former from an aqueous electroless plating solution containing a. a hypophosphite,

b. a nickel salt,

c. EDTA or one of its salts d. a pyrophosphate and e. an orthophosphate, as to produce a nickel phosphorus film on the former of the desired thickness and ii. heat stabilizing the resulting film resistor, the concentration of (0) being 0 to 0.09 g. moles/1, and that of (a), (b), (d) and (e) being sufficient to give, on complete dissociation into ions, 0.15 to 0.3 g. ions/l of hypophosphite, 0.01 to 0.1 g. ions/l of nickel, a pyrophosphate concentration between 5 and 15 times the difference between the concentration of metal and (c), an orthophosphate concentration of 0.01 to 0.06 g. ions/1, with the proviso that any (0) present should be at least 0.005 g. moles/l less than the ionic concentration of nickel in g. ions/l and the further proviso that amounts of the constituents in the bath must be such that the pH VALUE OF THE SOLU- TION REMAINS WITHIN THE RANGE 5-7. When EDTA (i.e. ethylenediaminetetraacetic acid) is present in the electroless plating solution used in the process of the invention, a suitable minimum amount is 0.005 g. moles/l.

EDTA is not an essential constituent of these solutions when very pure constituent chemical compounds are used. In practice it is often not possible to obtain adequately pure compounds with the result that precipitation occurs within the plating baths. EDTA minimizes precipitation or prevents it from occurring, e.g. by complexing with ionic impurities, and therefore plays an important role in ensuring the stability of the baths. It is therefore an essential constituent when impure chemicals are being used.

Although it is possible to deposit in the range of, e.g., room temperature to C., we have found that the best temperature for deposition is from 40 to 90 C., preferably around 50 C., but this can be varied, depending on the actual amounts of constituents in the bath. It will be appreciated that the precise quantities of the constituents in the bath will depend, inter alia, on the temperature at which deposition is effected. As the temperature for deposition is increased the faster-plating or least stable compositions progressively become unusable. Thus, the higher the temperature, the lower the optimum concentration of (a), (b), (d) and (e) and the higher the concentration of (c) if present.

The deposition time depends on the required surface resistivity of the nickel-phosphorus film and may be determined by simple experiment.

We have found that the TCR of the deposited films depends not only on solution pH value, concentration of hypophosphite and nickel salt, and solution temperature, but also on the known solutions, it was only generally possibleto obtain films with a TCR within the range of :50 p.p.m./C. when fairly acidic solutions were used. All other parameters of the process being equal, the lower value of surface resistivity desired, the lower is the pH value of the solution which gives the minimum TCR. For surface resistivities below ohms/square, for example, the optimum pH value for deposition from these previously known solutions is generally about 3.0. At such a low pH value electroless nickel films are continuously etched and dissolved as they are deposited, and we have found that this is a major factor which detracts from the uniformity of deposition of the film.

The undesirable etching is very'much slower when the solu mm. long steatite rods with a degree of surface roughness corresponding to a centerline-average index of 35 micro inches. The following activation procedure was used in every case.

One hundred rods are placed into a 250 ml. flask to which is then added a sufficient amount of an aqueous solution containing 1 percent weight/volume stannous chloride and l percent volume/volume hydrochloric acid to just cover the rods. The flask is then swirled from time to time during a period of 15 minutes. The excess solution is poured off the rods and they are thoroughly washed in distilled water. The rods are now covered with an aqueous solution containing 0.1 percent weight/volume of palladium chloride and 0.25 percent volume/volume of hydrochloric acid, and the flask is swirled gently for 2 minutes. The excess solution is poured off the rods and they are thoroughly washed in distilledwater of pH value between 6 and 8 for aperiod of about 5 minutes.

- In order to obtain the best possible uniformity of the activated surface, and to ensure a uniform start to the subsequent deposition process, the following reactivation" V procedure is used: Batches of 200 rods are placed in an aqueous solution containing 2 percent weight/volume sodium hypophosphite and 0.001 percent weight/volume nickel ions at the temperature of the plating bath of 2 minutes. This preheat solution is then decanted ofi and the rods immediately covered with the heated plating solution.

tion is neutral or only very slightly acidic, i.e. in the pH range from 5 to 7, but previously known plating solutions in this pH range are either unstable or they give such low phosphorous contents that they can only give films with a low TCR if the surface resistivity is very high.

In contrast to this general limitation on the choice of pH values for solutions, the process of the present invention allows a greater freedom of choice, the pH values being capable of being maintained within the range of from 5 to 7. The exact pH value chosen in this range is not important as the TCR of films obtained from these solutions is less dependent on the pH value than that of previously known solutions. When very low resistivity films are being prepared, thesolutions have a pH of 7, under which conditions the undesirable etching reaction, encountered with the low pH solutions previously used, is then minimized. Etching is less damaging to the thinner films required for higher resistivities and there is no significant advantage in this respect from the choice of pH values within the range of from 5 to 7 for high resistivity films.

As with previously known plating baths, the TCR of films deposited from the solutions used in our process varies as the resistivity is varied by altering the deposition time, all other conditions remaining constant. We have found, however, that the extent of this undesirable variation is dependent on the nature of the solution used. An unexpected advantage of our plating solution is that the extent of this variation is considerably smaller than in known solutions which have been examined. in consequence, the need to alter the deposition conditions according to the surface resistivity in order to obtain a minimum value of the TCR is considerably reduced.

Furthermore it has somewhat surprisingly been found that the TCR of films deposited from the solutions of the present invention at a pH value of 6 is slightly more positive than that of films of the same resistivity deposited at a pH value of 7, i.e. a decrease in pH value results in a more positive T.C. at a given resistivity value. This trend is the reverse of that obtained with previously used metallizing solutions.

Although the precision resistors of the present invention may be constituted by any suitable insulating former, it is preferred that the former should be of porcelain, steatite, or alumina or other ceramic (e.g. glass), but plastics (e.g. an epoxide, alkyd, phenolic or silicone resin) or other material known to be suitable as formers for precision resistors can also be used for making the former.

The Examples given hereinafter describe the deposition of a nickel-phosphorous alloy on clean 2.5 mm. diameter by 10 Plating and Stabilizing Example l A solution is prepared by dissolving Niso, mp 25 g. NaH PO, ",0 25 g. Na,l',0, 10H,o so g. EDTA disodium salt 30 g. Nr HPo, |2H,o 73.

in distilled water and diluting to l litre. The pH value of this a value of surface resistivity which is required. For example, a plating time of 25 minutes results in films having a surface resistivity of i0 ohms/square. The solution is poured off and the plated rods are washed thoroughly in running tap water. They are then rinsed in distilled water and finally in acetone. They are dried at room temperature in air. The rods are now stabilized by spreading them out on flat trays and heating them for 5 hours in a clean oven at 250 C.

The dependence of the T.C.R. on surface resistivity for films which are obtained by this procedure where only the duration of the deposition is altered to give the different resistivities, is illustrated by curve A shown in HO. 1 of the accompanying diagrammatic drawing. Typical deviations from this curve within a batch of rods are small, i.e. of the order of :20 p.p.m./ C., when manufacturing conditions are carefully controlled and reproduced as described above. lt can be seen that the mean T.C.R. is zero at about 200 ohms/square, +40 p.p.m./C. at about 7 ohms/square and 40 p.p.m./C. at approximately 800 ohms/square.

The greater variation of TCR. with surface resistivity of solutions previously used for the chemical deposition of resistive films of a nickel-phosphorous alloy is exemplified by curve 8" in FIG. 1 of the drawings. This curve shows the results obtained when the conditions of preparation are the same as those described in example i, except that the following previously known solution is used:

NiSO, 7H,O 29 g.ll

Succinic Acid 7.6 gl] NaOH added to give a pH of 3.7.

The importance of having as small a variation as possible of the T.C.R. for a given variation of surface resistivity can be seen from the following: Theprocedure which utilizes the present invention and gives the results represented by curve A" and also the previously known procedure which gives the results represented by curve 13" are both particularly suited for the preparation of films having a surface resistivity of about 200 ohms per square, since the T.C.R. is at a minimum around this value. However, for any other value of surface resistivity, it can be seen that the T.C.R. given by curve B is always numerically greater than that given by curve A. One reason for preferring the processes which give the smallest variation of T.C.R. with resistivity is that when a batch of rods is being plated small variations from one rod to another in parameters such as plating time almost inevitably occur, resulting in variations in film thickness. For example, it is almost impossible to bring the metallizing solution into contact with all the rods at exactly the same moment in time. All the rods in a batch do not therefore have exactly the same surface resistivity. Although this spread in resistance value is not necessarily undesirable in itself, it is obviously important that a large proportion of the batch should have the desired T.C.R. The possible yield of resistors within the required limits of T.C.R. obviously depends on the magnitude of the variation of T.C.R. with resistance. Similarly, any deviation of the mean plating time of the bath from the intended value results in a deviation of the mean surface resistivity of the batch from the intended one, at which the T.C.R. is an optimum. The smaller variation of T.C.R. with resistivity value which is obtained when the present invention is used is therefore of great importance when a high yield of resistors with a small T.C.R. (e.g. within :50 p.p.m./C.) is required.

If T.C.R. limits better than :50 p.p.m./C. are required, the use of the above solution containing EDTA and phosphates is restricted to the preparation of a narrow range of surface resistivities, centered around 200 ohms/square, i.e. centered around the value at which the mean T.C.R. is zero.

Examples 2 and 3 below show typical modifications which can be made to the formulation of example 1 to give an optimum T.C.R. either at a lower or at a higher value of surface resistivity than 200 ohms/square. The modifications which make the T.C.R. more positive, and hence more suitable for plating films of high surface resistivity, can be achieved by the use, within the limits specified above, of high concentrations of nickel and of phosphates, or low concentrations of hypophosphites and EDTA. 7

.Suitable compositions within these specified limits can also be chosen to ensure adequate solution stability.

Example 2 A solution is prepared by dissolving NiSO, 7H,0 26 g. NaH,P0, ",0 25 g. Na,l,0, lH,0 49 g. EDTA disodium salt 27 g. Na,HPO, I2H,O 2.9 g. Na,PO, l2H,O 3.7 g.

in distilled water and diluting to l litre. The pH value of this solution is 6. 100 ml. of this solution is heated to 50 C. and transferred to the heated activated rods and the plating is allowed to proceed for a period which depends on the exact value of surface resistivity which is required. The rods are washed and stabilized as described in example 1. A plating time of just under 2 minutes is required to give a mean surface resistivity of 2 kilohms/square and the T.C.R. is numerically less than that which is obtained at this surface resistivity using the solution described in example 1. A good yield of resistors can be obtained having a T.C.R. with 0 p.p.m./C., or even closer limits depending on the accuracy with which the process is controlled. Example 3 For the deposition of low value films it is preferable to use a neutral solution, i.e. a solution with a pH value of 7.

A solution is prepared as described in example 1 and the pH is adjustedto a value of 7 using sodium hydroxide. 200 ml. of this solution is heated to 50 C., transferred to the heated activated rods and the plating allowed to proceed for a period which depends on the exact value of surface resistivity which is required. The rods are washed and stabilized as described in example 1. A plating time of 10 minutes results in a means surface resistivity of 10 ohms/square with a T.C.R. which is slightly less positive (by about 20 p.p.m./C.) than that which would be obtained at this surface resistivity using a solution with a pH value of 6. A good yield of resistors having a T.C.R. within 0 p.p.m./C. is generally obtained and very accurate control of the conditions of preparation can result in a good yield within even smaller T.C.R. limits, e.g. :20 p.p.m./C. Processing of metallized rods After stabilizing, these films are processed into finished resistors ready for use in the usual way by fitting terminals and patterning to the required value and then coating with an organic protection, using either a stovable lacquer or a thermoplastic or thermosetting moulding. These organic jackets serve to protect the film against mechanical and chemical damage and in addition insulate the resistor.

FIG. 2 of the accompanying diagrammatic drawing shows a resistor of the present invention in cross sectional view. Reference numeral 2 designates a ceramic rod which is submitted to the required cleaning and activation process and then metallized following the manner of the present invention. Reference numeral 3 designates a metal alloy film of nickel and phosphorus which has been electrolessly deposited over the surface of rod 2; end caps 4 and terminal leads 5 are provided to afford electrical connection to the resistive film.

Although the present invention is described herein with particular reference to specific details, it is not intended that such details shall be regarded as limitations upon the scope of the invention except insofar as included in the accompanying claims.

1. A process for the production of a nickel-phosphorus film precision resistor which comprises:

i. providing an aqueous electroless plating solution having a pH value of from 5 to 7 by dissolving therein A. the required materials selected from the class consisting of electroless plating solution ingredients comprising (a) hypophosphites, (b) nickel salts, (c) EDTA and salts thereof and (d) pyrophosphates, said electroless plating solution ingredients being chosen in such a way that the concentration of (c) is 0 to 0.09 g. moles/l and that of (a), (b) and (d) is sufficient to give, on complete dissociation into ions, 0.l5 to 0.3 g. ions/ l of hypophosphite, 0.01 to 0.1 g. ions/l of nickel, a pyrophosphate concentration between 5 and 15 times the difference between the concentration of metal and (c), and

B. a water soluble orthophosphate chosen in such a way as to give, on complete dissociation into ions, a concentration of 0.01 to 0.06 g. ions/l, with the proviso that any (c) present should be at least 0.005 g. moles/l less than the ionic concentration of nickel in g. ions/ 1 ii. effecting deposition on an activated insulating former from said aqueous electroless plating solution so as to produce a nickel phosphorus film on the former of the desired thickness and,

iii. heat stabilizing the resulting film resistor.

2. A process according to claim 1, in which constituent (c) is present in a concentration of 0.005 to 0.09 g. moles/ l.

3. A process according to claim 1, in which the plating temperature is from 40 to C.

4. A process according to claim 1, in which the aqueous electroless plating solution is a solution resulting by dissolving in distilled water per litre of solution the following ingredients or their chemical equivalents:

nickel sulfate approximately 25 g.,

water soluble hypophosphite approximately 25 g.,

water soluble pyrophosphate approximately 50 g.

water soluble salt of EDTA approximately 30 g. and water soluble orthophosphate approximately 7 g.

5. A process according to claim 1, in which the aqueous electroless plating solution is a solution resulting by dissolving water soluble hypophosphite approximately 25 3., water soluble pyrophosphate approximately 49 g., water soluble salt of EDTA approximately 27 g.

and water soluble orthophosphate approximately 6.6 g. 

2. A process according to claim 1, in which constituent (c) is present in a concentration of 0.005 to 0.09 g. moles/1.
 3. A process according to claim 1, in which the plating temperature is from 40 to 90*C.
 4. A process according to claim 1, in which the aqueous electroless plating solution is a solution resulting by dissolving in distilled water per litre of solution the following ingredients or their chemical equivalents: nickel sulfate approximately 25 g., water soluble hypophosphite approximately 25 g., water soluble pyrophosphate approximately 50 g. water soluble salt of EDTA approximately 30 g. and water soluble orthophosphate approximately 7 g.
 5. A process according to claim 1, in which the aqueous electroless plating solution is a solution resulting by dissolving in distilled water per litre of solution the following ingredients or their chemical equivalents: nickel sulfate approximately 26 g., water soluble hypophosphite approximately 25 g., water soluble pyrophosphate approximately 49 g., water soluble salt of EDTA approximately 27 g. and water soluble orthophosphate approximately 6.6 g. 